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Wastewater Treatment

Plants


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Thought of the Day

Run after Sarasvati

and Laxami will follow you(Hindi Proverb)

Sarasvati: The Goddess of KnowledgeLaxami: The Goddess of Fortune

Wastewater Treatment Plants 2


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Primary, Secondary and TertiaryTreatment



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Why Treat Wastewater?

why such treatment is needed

the nature of the materials which are

removed from wastewater explore the processes that accomplish

that removal

consider wastewater treatment in cities in small communities

at individual homes



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Why Treat Wastewater?

Domestic and industrial wastes are treated before beingdischarged to rivers and lakes

to protect the wildlife that call those environments home modern technology offers remarkable capabilities to

accomplish these goals introduction of new and different pollutants cost of treating to required levels is a challenge for the

wastewater treatment industry in some cases, wastewater can be clean enough for reuse

for particular purposes wastewater treatment systems use the same processes of

purification that would occur in a natural aquatic systemonly they do it faster and in a controlled situation



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Sewage or Wastewater

Sewage or wastewater isfrom:

Domestic used water andtoilet wastes

Rainwater Industrial effluent (Toxic

industrial water ispretreated)

Livestock wastes Commercial activities (e.g.

laundries, gas stations) Institutions



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Total Maximum Daily Load(TMDL)

Water quality standard are set by theEnvironmental Protection Agency (EPA) toprotect uses for surface water

If the standards are not met, then acalculation is made to determine how muchof a pollutant can be discharged to thelake or river without exceeding thestandards

This amount is called the Total MaximumDaily Load (TMDL)



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Wastewater - What's In It?

Pathogens: disease-causingorganisms

Solids: suspendedparticles that make thewater turbid

Nutrients: such asphosphorus that can causenuisance plant growth

Organic matter: that canrob the oxygen resourcesof streams

Toxics: metals andsynthetic organicchemicals harmful tohumans and wildlife



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Wastewater



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Wastewater - Where Does It Go

Wastewater is carried to thetreatment plant via sanitarysewers

The connection from thehouse to the sewer system isa pipe, about 6 in diameter

A larger pipe (8-12 indiameter), called a collectingsewer, runs down the streetpicking up the wastewaterfrom individual sewers

These pipes eventually run toan interceptor sewer whichdelivers the flow to thetreatment plant

In large cities, interceptorsewers can be huge,sometimes big enough todrive a car through!

Engineers design sewersystems to take advantage ofgravity flow, but sometimespumping is required

Rainwater is collected bystorm drains and flowthrough storm sewers tosettling ponds or lakes and

rivers



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Combined Sewer System



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Combined and Separate Sewers

In older cities, the sewers carry both wastewater andrainwater that runs off of the streets

These are called combined sewers During heavy rain events, the flow in combined sewers can

reach capacity and threaten to back up into your house To prevent this, relief structures called combined sewer

overflows (CSOs)are constructed sending the excesswater into streams and lakes

CSOs can grossly pollute rivers and lakes and thus effortsare being made to limit their discharge

Today, separate sewers are installed, with rain runoffdischarge to surface waters via storm sewers andwastewater routed to the treatment plant with nodischarge on the way



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Wastewater TreatmentOverview

Everything that happens in a wastewatertreatment plant would happen in nature anyway

Pathogens would die Solids would settle to the lake bottom Nutrients would be removed from the water by

plants Organic matter would decompose Toxic materials would eventually move into the

air or into the bottom sediments of a river orlake



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Basic Steps in WastewaterTreatment

The basic steps inwastewater treatment are: preliminary treatment -

to protect plant

operation settling - to remove

solids biological treatment - to

remove organic matter

disinfection - to removepathogens solids processing - to

provide safe finaldisposal



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Preliminary Treatment

Preliminary treatmentseeks to remove rags and solids that

float which may harmthe operation of therest of the plant

Bar racks or mechanicalscreens remove paper,rags and other largesolids

Sand and grit areremoved by gravitysettling in a gritchamber



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Primary Treatment

Following preliminary treatment, thewater moves on to a clarifier or settlingtank

This is primary treatment

Sewage solids settle to the bottomof the tank

Not all of the solids are removedhere in the primary clarifier becausevery small, light particles take toolong to settle

These small particles and dissolvedorganic matter are removed later

The solids that collect at thebottom of the tank arecalled primary sludge and arepumped from the tank for treatmentelsewhere in the plant



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Secondary TreatmentActivated Sludge

Microorganisms such as bacteriaand protozoa can use the smallparticles and dissolved organic matter,not removed in primary treatment, asfood

Secondary or biological treatment isperformed in a tank containing a "soup"

of starved microbes called activatedsludge These microbes require air to live

(they are aerobic organisms) and thusair is pumped into the tank

Microorganisms in this aerationtank use the dissolved and particulateorganic matter as food, producing more

microorganisms Thus the waste materials that left theprimary clarifier are changed intomicroorganisms which can be collectedand separated from the water in thenext step



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Secondary TreatmentClarifier

It then remains to separate outthe microorganisms (activatedsludge) so that just clean wateris left

This is done in a secondaryclarifier which operates in the

same manner as the primaryclarifier described previously Some of the solids collected in

the secondary clarifier (returnactivated sludge) are sent backto the aeration tank to treatmore wastewater and the excess

(waste activated sludge) ispumped to another location inthe plant for further treatment

The clean water that flows outthe top of the clarifier is sentalong for disinfection



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Disinfection The final step before releasing

the treated wastewater to theenvironment is to killmicroorganisms that might behealth hazards

This process, called disinfection,can be accomplished byadding chlorine or by exposingthe wastewater to ultravioletlight(UV)

Disinfection with chlorine canproduce chemicals which aredangerous to public health, soalternatives such as ultravioletlight are becoming more popular

The treated wastewatereffluent can now be dischargedto rivers or lakes



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Solids Management

We have removed solids at variouspoints in the wastewater treatmentprocess

primary sludge composed of thesewage solids collected in theprimary clarifier

waste activated sludge from thesecondary clarifier

This sludge presents two problems

First, it is rich in organic matter andwill consume oxygen and create foulodors if delivered to the

environment without furthertreatment

Second, even though it's calledsludge, it is mostly water and isdifficult to handle in final disposal

Thus sludge processing involvestwo steps:1. digestion - to break down

the organic matter2. dewatering- to reduce the

water content and make iteasily handled

The treated sludge is then senton for final disposal



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Wastewater Treatment



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Digestion

Organic matter is brokendown and made inert in aprocess called anaerobicdigestion

Specialized bacteria, able

to live without oxygen(anaerobic conditions), usethe biodegradable organicmatter as a food source

Producing carbon dioxideand methane gas and inert

organic matter



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Digestion Process Anaerobic digestion is a two-step

process In the first step, bacteria and sludge

are mixed together in a heated tankcalled a digester

This is where the biological activitytakes place and biodegradable organicmatter is broken down producing gasesand inert organic matter

The sludge is then delivered to thesecond step

This digester is not mixed and thesludge separates with gases at the topof the digester, liquid in the middleand solids on the bottom

Gases that have risen to the top are

collected and burned The liquid is returned to the beginning

of the plant to be treated again Solids are sent to the next step for

dewatering



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Dewatering There are several different

ways to remove water fromsludge

One of these is called a beltfilter press

Sludge is placed between two

cloth belt filters that pass overa series of drums or rollerssqueezing out the liquid

At the end of the press, a knifescrapes the sludge cake off ofthe belt filter

Liquid removed during pressing

is returned to the beginning ofthe plant for further treatment The sludge cake is then sent on

for final disposal



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Final Effluent

Influent is water that "flowsin

This is the raw, untreatedwastewater

Effluent means to "flow out

This is the treatedwastewater The picture at right

compares influent andeffluent

More than 95% of the

pollutants have been removed This water is safe fordischarge to lakes or rivers



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Wastewater Treatment Plant

Kasur



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Settling Tanks



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Inflow



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Screening



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Aerators



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Aerators



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Sedimentation Basin



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Influent vs Effluent



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Islamabad


l


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Wastewater Treatment PlantModel



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Screening



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Aeration



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Aeration



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Sedimentation Basin



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Supernatant Water



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Sludge



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Dry Sludge



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Nestle



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Aeration



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Suspended Solids



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pH & Temperature



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Sedimentation Tank



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Automatic Feeding



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Scrappers



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Under Construction



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Automation Drive



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Fully Automated



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Final Effluent



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Advanced Wastewater Treatment



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Thank You

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